This invention relates to a lightweight composite pressure vessel, such as a filament wound pressure tank. A flat bottom to a pressure vessel is provided without sacrificing the weight savings inherent in the composite design.
It is common practice for pressure vessels in the form of industrial gas cylinders to be manufactured with flat bottoms enabling the cylinders to stand upright for filling and general use. These industrial gas cylinders typically contain various gases under pressure (argon, acetylene, nitrogen, oxygen, etc.). The vast majority of these cylinders are constructed from steel and aluminum.
In weight critical applications, aluminum cylinders have dominated steel in recent years for market share. One example is an oxygen cylinder for the medical industry. However, composite cylinders offer an even greater weight savings and have the highest performance factor as compared to steel and aluminum. Performance factor is defined as PV with P=operating pressure, V=internal volume (water capacity), W=weight of vessel at 0 psig.
Although steel and aluminum cylinders have flat bottoms designed into them to allow them to stand upright, they have low performance factors. Contributing to their poor performance factor (heavy cylinder design) is the design philosophy of the flat bottom itself. By exposing the flat bottom (as typical in industrial gas cylinder design) to the internal pressure, a massive thickness of material is required to resist the high stresses caused by the flat geometry. This results in an extremely heavy cylinder, with the majority of the weight being in the design of the bottom itself. This phenomenon is demonstrated through the simple stress equation for a homogeneous material membrane stress for pressure vessels .rho.=PR, with .sigma.=stress, T=thickness of material, and R=radius of curvature. As one can see as the radius of curvature gets larger (R.fwdarw..infin., for flat geometry) the stresses increase dramatically therefore, T=thickness must increase alto to keep the .sigma.=stress reasonably below the material allowable stress.
While composite cylinders seem the logical choice in the medical industry to replace aluminum, the market share has been extremely limited at best. The limited use of composite cylinders in this industry is due to several factors. One major factor is the lack of a flat bottom on existing composite cylinder designs. There exists a need for a flat bottom without sacrificing the structural efficiency already known and established in the art of composite pressure vessel design and manufacturing. Such devices must have sufficient strength and durability to support the vessel in the upright condition regardless of vessel L/D ratio and must allow the complete exterior to be visually inspected by Department of Transportation (D.O.T.) retesters. Such a device should make composite pressure vessels available widespread use in applications where flat bottoms are a requirement, such as the medical industry.